1. Field of the Invention
The invention relates to power supplies in general and more particularly to power supplies that are used to provide voltages for video devices or the like.
2. Prior Art
Video display devices, such as television receivers and computer monitors incorporate power supplies that produce one or more regulated voltage levels for powering various load circuits. The load circuits may be incorporated in the video display devices or may be peripheral or external devices. In a typical arrangement the power supply includes a power transformer which has a primary winding and a secondary winding. The primary winding is coupled through a switching transistor sub-system to an energy source and the secondary winding is coupled through regulating circuits to the display. The switching transistor sub-system includes one or more sense/control circuits which result in regulation of the output voltages.
Occasionally, error or fault conditions may occur in the power supply and/or the load circuits. If not protected, such error conditions could damage the power supply and/or load circuits. One way to protect against such damage is to provide protection circuitry that shuts down the power supply on the occurrence of a fault condition. Because the circuits that control the switching transistor are usually located on the primary side of the power transformer, the protection circuitry is usually located there. However, in order to protect circuits, devices, etc. on the secondary side, protection circuits are also required there. As a result the problem is how to send information from the fault detection circuits on the secondary side to the primary side without losing the cost advantages of primary sensing and without adding any new components which would necessarily have to meet the primary to secondary isolation requirements.
One conventional solution to this problem is to use a transformer or optical isolator to interconnect fault detection circuits on the secondary side of the power transformer with the protection circuitry on the primary side. An example of this solution is set forth in U.S. Pat. No. 4,709,321. Even though this solution works well for its intended purpose, it adds a relatively expensive component which is unacceptable for cost sensitive display products.
Another solution is to intentionally short circuit an output or secondary side voltage when a fault condition is detected. This causes an overcurrent condition in the primary side circuit. This overcurrent condition is sensed and is used to trigger the shutdown circuit. An example of this technique is set forth in U.S. Pat. No. 4,685,020. This approach is a step in the right direction in that it requires no pulse transformer or optical isolators. Instead, the fault information is passed through the power transformer in the form of an overcurrent. One possible disadvantage is that circuitry must be added to create the secondary short circuit. The circuitry usually includes a power silicon control rectifier (SCR). SCRs are usually expensive and further increase the cost of the power supply.